This invention relates to a polishing apparatus which uses a running and rotating tape to abrade or clean a substrate, and more particularly a flat substrate such as a liquid crystal glass substrate or a printed circuit board.
Liquid crystal products are being used in all fields of application. Liquid crystal display devices are particularly common for word processors, personal computers and television sets and are produced by carrying out various processes on glass substrates, cutting them, injecting a liquid crystal material between two of them, sealing them and attaching a polarization plate. During such a series of production processes and, in particular, when glass plates are cut, a liquid crystal material is injected or the liquid crystal is sealed in, contaminants such as glass scraps (or cullet) or residues of a resin material used for the sealing of the liquid crystal are generally generated. Such contaminants have been known to be a source of defective products.
These contaminants used to be removed by a manual labor, such as by using a paring knife or a solvent such as acetone or an alcohol. Manual removal of contaminants, however, tends to adversely affect the productivity of liquid crystal display devices and also makes it difficult to obtain products with a uniform quality level.
Electronic apparatus such as word processors and personal computers make use of printed circuit boards for attaching electronic elements such as semiconductor chips. When such circuit boards are produced, both chemical processes such as formation and removal of thin films and physical processes such as drilling and cutting are carried out in order to form circuit lines on a board. For this purpose, both surfaces of the substrate must be abraded and polished, and it has been known to make use of an endless belt, or to carry out etching, sand-blasting and buffing operations. Use of abrading tapes is becoming popular because abrading tapes can be used effectively for polishing and they can be handled easily.
U.S. Pat. No. 5,569,063 disclosed a polishing apparatus for abrading and cleaning a target object while an abrading tape is run and rotated. This apparatus has a box-shaped member attached at a lower end of a spindle adapted to rotate around its vertical axis of rotation by means of a motor such that this box-shaped member can also rotate around the axis of rotation of the spindle. A compression roller is horizontally disposed at the lower end of this box-shaped member, which also contains therein a supply roller and a take-up roller with their axes of rotation kept horizontally and parallel to each other such that an abrading tape supplied from the supply roller passes around the compression roller and is taken up by the take-up roller. The spindle and the box-shaped member are rotated by the motor while the abrading tape is supplied and taken up. Thus, the abrading tape rotates while it advances longitudinally, or "runs" and hence it can effect an abrading and polishing operation efficiently.
At the beginning of an abrading or polishing operation, the unused part of the abrading tape is mostly wound around the supply roller and the take-up roller is nearly completely empty. As the operation continues, the amount of the tape around the supply roller diminishes while the amount of the tape wound around the take-up roller increases. Thus, the center of gravity of the polishing apparatus gradually shifts from somewhere closer to the supply roller towards the take-up roller.
For effecting a smooth rotary motion, the center of gravity of the apparatus should be exactly on the axis of its rotation. In the case of the apparatus described above, however, the rotary motion is not stable because its center of gravity is near the supply roller in the beginning and gradually moves towards the take-up roller as the polishing operation continues. When the apparatus is rotated at a fast rate, in particular, the center of rotation will oscillate significantly and this may even affect the useful lifetime of the apparatus adversely.
In order to effect a smooth polishing operation, furthermore, not only should the rotary motion be smooth but it is also desirable that the running (translational) speed of the tape be constant. If the running speed is too low, this means not only that the speed of polishing is slow but the part of the tape which has been used and has hence become less effective is still being used, and this affects the efficiency of polishing adversely. If the running speed is too fast, on the other hand, although the speed of abrasion is increased and the polishing efficiency is increased because an unused part of the tape with high effectiveness is always being used but a part of the tape which has not been fully used up and hence can still contribute to the abrasion work is constantly being taken up and this amounts to a waste of a useful resource.